The integrated conjugative plasmid pSAM2 of Streptomyces ambofaciens is related to temperate bacteriophages.

Abstract

Streptomyces ambofaciens ATCC23877 and derivatives contain the 11-kb element pSAM2 present in an integrated state or as a free and integrated plasmid. This element, able to integrate site-specifically in the genome of different Streptomyces species, is conjugative and mobilizes chromosomal markers. Besides these plasmid functions, we have shown that the site-specific recombination system of pSAM2 presents strong similarities with that of several temperate phages. The integration event is promoted by a site-specific recombinase of the integrase family. The int gene encoding this integrase is closely linked to the plasmid attachment site (attP). A small open reading frame (ORF) overlaps the int gene and the predicted protein exhibits similarities with Xis proteins involved in phages excision. The integrated copy of pSAM2 in strain ATCC23877 is flanked by att sequences (attL and attR). Another att sequence (attX) is present in this strain and attX and attL are the boundaries of a 42-kb fragment (xSAM1) absent, as well as pSAM2, from S.ambofaciens DSM40697. Sequences partially similar to pSAM2 int gene are found near the chromosomal integration zone in both S.ambofaciens strains. The possible origin of pSAM2, an element carrying plasmid as well as phage features, is discussed.